In kraft cooking, wood is delignified by a cooking liquor where the active components are Na.sub.2 S and NaOH. Na.sub.2 S is preferably primarily active at the beginning of the cook, and NaOH is preferably active at the end of the cook. The invention relates to a method of increasing Na.sub.2 S concentration in the beginning of the cook. A high Na.sub.2 S concentration in the beginning of the cook gives a more selective cook and the possibility of lowering pulp kappa.
Spent kraft cooking liquor, or black liquor, has been re-circulated for re-use in various fashions in prior art continuous kraft cooking systems. For example in U.S. Pat. No. 3,802,956, black liquor is added to the feed system of a continuous digester to aid in flushing chips from the high pressure transfer device (i.e., the high pressure feeder) to the top of the impregnation vessel. In U.S. Pat. Nos. 5,080,755 and 5,192,396 black liquor is used to supplement the liquor extracted in the impregnation vessel. These patents disclose a method of introducing co- and counter-current chip impregnation in an impregnation vessel by extracting liquor at a midpoint in the so vessel. The re-circulated black liquor is used to increase the liquor volume (i.e., the liquor-to-wood ratio) to limit the alkali concentration increase caused by the extraction. The black liquor also improves the chip column movement.
In the early 1980s, based upon work performed by Sjoblom et al at the Swedish Royal Institute of Technology, it was recognized that the presence of sulfides in the early stages of kraft cooking can improve the strength of the resulting pulp. Attempts have been made to use the sulfides present in black liquor to provide the sulfide desired. U.S. Pat. Nos. 5,053,108 and 5,236,553 disclose a method of continuous kraft cooking in which black liquor is re-circulated to the feed system to treat the wood chips. In the '108 patent this treatment takes place in the chip chute and is referred to as "sulfonation". In the '553 patent, the black liquor is re-circulated to a chip chute/slurrying vessel. In both patents a single source of black liquor is used.
Recent mathematical modeling pursuant to the invention suggests that a preferred method of pre-treating chips is by using two forms of black liquor: one of relatively weak sulfide concentration and one of relatively strong sulfide concentration. The invention provides such two different streams in a practical manner.
According to one aspect of the present invention, a method of continuously kraft cooking comminuted cellulosic fibrous material utilizing an upright continuous digester having first and second extraction screens, a top, and a bottom, is provided. The method comprises the steps of continuously: (a) Feeding comminuted cellulosic fibrous material slurry to the top of the digester. (b) Cooking the material in the digester as it passes downwardly therein, at a cooking temperature of about 150.degree.-180.degree. C., producing spent liquor during cooking while digesting the material. (c) Using the first so extraction screen, withdrawing a first spent liquor from the digester having a first Na.sub.2 S concentration (and typically a first amount of effective alkali). (d) Using the second extraction screen, withdrawing a second spent liquor from the digester having a second Na.sub.2 S concentration (and typically a second amount of effective alkali) at least 25% less than the first Na.sub.2 S concentration (and typically at least 25% less than the first amount of effective alkali). (e) Combining at least some of the first spent liquor with the material prior to step (b) to enhance the amount of sulfur present at the beginning of step (b). And, (f) withdrawing pulp from the bottom of the digester.
In the practice of the method described above, step (c) is typically practiced to extract as the first spent liquor a liquor having an effective alkali concentration of about 10-50 g/l and an Na.sub.2 S concentration (assuming original sulfidity of the cooking liquor of greater than about 25%) of greater than about 15 g/l, possibly greater than about 35 g/l. The first liquor is also typically at a temperature of about 140.degree.-170.degree. C. and in an amount of about 2-6 m.sup.3 /ton of pulp (t.p.). The extracted liquor may be treated to improve its usefulness in the impregnation and cooking. Suitable treatments are, e.g. flashing to separate gases and raise concentration, evaporation to raise concentration, separation of organics like lignin to lower dry solids content, heating to change the structure of organic material and sulphur, raising or lowering the temperature, and filtration.
Step (d) of the above method is typically practiced to extract as the second spent liquor a liquor having an effective alkali concentration of about 3-20 g/l (typically half or less of the concentration of the first liquor), and an Na.sub.2 S concentration of less than about 20 g/l (typically less than half of that of the first liquor). The second liquor typically has a temperature of about 120.degree.-160.degree. C. and is extracted in an amount of about 2-6 m.sup.3 /t.p. The extracted liquor may be treated to improve its usefulness in the impregnation and cooking. Suitable treatments are, for example, flashing to separate gases and raise concentration, separation of organics like lignin to lower dry solids content, heating to change structure of organic material and sulphur, raising or lowering temperature and filtration.
The digester may comprise a two vessel hydraulic system including a first impregnation vessel, in which case step (e) may be practiced to introduce the first spent liquor into the bottom of the impregnation vessel to flow co-currently or countercurrently to the material therein, or the digester may comprise a single vessel hydraulic digester in which case the first liquor may be introduced into the slurry co- or countercurrently any time prior to the practice of step (a).
The second spent liquor may be flashed to steam in a flash tank, and a more concentrated second spent liquor which is withdrawn from the bottom of the flash tank may be used to slurry the comminuted cellulosic fibrous material prior to step (a). For example, where a high pressure transfer device is provided (high pressure feeder) having a feed circulation loop, which transfer device feeds slurry to the top of the digester (either directly or through an impregnation vessel), the more concentrated second spent liquor may be introduced into contact with the material in the feed circulation loop of the high pressure transfer device, i.e. slurrying the material. The first liquor may also be flashed if desired, before adding it to the cellulosic material upstream of the digester cooking zone. Such a flashing may be useful to separate gases that otherwise could disturb the operation of the system. Another reason to flash is to lower the temperature if this is found necessary.
In order to enhance further the amount of sulphur present at the beginning of the cook, there may be the further step of supplementing the first spent liquor with just above 0 to about 2 m.sup.3 /ton of pulp of green liquor. Other liquors that can be added are white liquor or sodium hydroxide. By the addition of green liquor, white liquor and sodium hydroxide the sodium-sulphur balance can be adjusted.
Additive chemicals like polysulfide and anthraquinone can be used in this process. For example the polysulfide should be added to the pretreatment phase to raise sulfide ion concentration. Anthraquinone is a catalyst that is only partly consumed during cooking. By the recirculation system a recirculation of anthraquinone is obtained lowering the usage of fresh costly anthraquinone. Other additives to the cooking process, for example, chelating agents, such as EDTA, can be used in a similar manner.
According to another aspect of the present invention a method of continuously digesting comminuted cellulosic fibrous material to produce cellulose pulp comprises the steps of substantially consecutively and continuously: (a) Slurrying the material with liquor. (b) Treating the material with a first sulphurous liquor having an effective alkali concentration of about 10-50 g/l and an Na.sub.2 S concentration of at least about 15 g/l (e.g. 15-60 g/l), and possibly at least 35 g/l (e.g. 40-60 g/l), although preferably about 20-30 g/l. (c) Cooking the material by adding a cooking liquor having an effective alkali concentration of over 100 g/l and a sulfidity of at least about 25% at a cooking temperature of about 150.degree.-180.degree. C. (e.g. 150.degree.-175.degree. C.) to produce pulp. (d) Separating the first sulphurous liquor from the pulp. (e) Separating a second liquor from the pulp having a different effective alkali concentration and Na.sub.2 S concentration than the first liquor. And, (f) washing the pulp.
In the practice of the above method, step (a) may be practiced, at least in part, using the second liquor from step (e). The first and second liquors preferably have the temperature ranges and volumes described above with respect to a first aspect of the present invention. The second liquor typically has an effective alkali concentration of about 3-20 g/l (e.g. about 10 g/l) and less than the effective alkali concentration of the first liquor, and has an Na.sub.2 S concentration of less than about 20 g/l (e.g. about 5-15 g/l).
According to another aspect of the present invention a continuous digester system is provided. The continuous digester system comprises the following elements: An upright digester vessel having a top and a bottom. A chip slurry feed inlet adjacent the top of the vessel. A chip feed system connected to the chip feed inlet. A pulp outlet adjacent the bottom of the vessel. A separating device adjacent the top of the vessel for separating some liquor from chips fed into the chip feed inlet and returning it to the chip feed system. At least one upper screen in the vessel distinct from the separating device. A first extraction screen in the vessel below the at least one upper screen for extracting a first spent liquor. A second extraction screen in the vessel below the first extraction screen for extracting a second spent liquor distinct from the first spent liquor. And, a first conduit for circulating the first spent liquor to the chip feed system.
In the continuous digester system described above, a chip slurrying system is also preferably provided connected to the chip feed system opposite the digester. A second conduit is also provided for circulating liquor from the second extraction screen to the slurrying system. The second conduit may be connected directly to the slurrying system, or through one or more flash tanks.
The invention also may include a simplified steaming and so slurrying system associated with the digester. For example instead of utilizing a chip bin, chip meter, chip feeder, horizontal steaming vessel, slurrying vessel, and high pressure feeder, the steaming and slurrying system may consist essentially of only a chip bin, chip feeder, slurrying vessel (e.g. chute), and high pressure feeder.
The continuous digester system may comprise any of a wide variety of conventional digester systems including digesters available from Kamyr, Inc. of Glens Falls, N.Y. and sold under the trademarks MCC.RTM., EMCC.RTM., and LO-SOLIDS.TM.; the continuous digester system may include a single vessel hydraulic system, a two vessel hydraulic system (with an impregnation vessel in addition to the digester), or other conventional systems. Where an impregnation vessel is utilized, the first conduit may be connected to the impregnation vessel to introduce the first spent liquor into the impregnation vessel. In the impregnation vessel the first, sulphurous, liquor typically flows countercurrently to the cellulosic material, but may instead be directed to flow co-currently.
According to still another embodiment of the invention a method of continuously kraft cooking comminuted cellulose material is provided, comprising the following steps: (a) Treating the material with a first black liquor for at least 10 minutes at a temperature between 80.degree.-110.degree. C. (b) Treating the material with a second black liquor for at least 10 minutes at a temperature between 110.degree.-140.degree. C. (c) Adding cooking liquor to the material, and cooking the material at a temperature between 150.degree.-180.degree. C. producing black liquor during cooking of the material. (d) Withdrawing the second black liquor, from the digester, having a concentration of sulfide ions, and using at least part of the second black liquor during the practice of step (b). And, (e) withdrawing the first black liquor from the digester having a concentration of sulfide ions lower than that of the second black liquor, and using at least part of the first black liquor during the practice of step (a).
It is the primary object of the present invention to provide two or more different streams of spent cooking liquors so as to provide a high Na.sub.2 S concentration in the beginning of a kraft cook, resulting in a selective cook, better pulp strength and the potential to lower cooking kappa (i.e. easily below 20), and a simplified chip feeding system. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.